Volume 538, February 2012
|Number of page(s)||18|
|Section||Galactic structure, stellar clusters and populations|
|Published online||27 January 2012|
Two distinct halo populations in the solar neighborhood
Observatorio Astronómico Nacional, Universidad Nacional Autónoma de
México, Apartado Postal
877, CP 22800
2 Instituto de Astronomía, Universidad Nacional Autónoma de México, AP 70-264, 04510 México, DF, México
e-mail: email@example.com; firstname.lastname@example.org
3 Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
Accepted: 2 November 2011
Context. In Papers I and II of this series, we have found clear indications of the existence of two distinct populations of stars in the solar neighborhood belonging to the metal-rich end of the halo metallicity distribution function. Based on high-resolution, high S/N spectra, it is possible to distinguish between “high-alpha” and “low-alpha” components using the [α/Fe] versus [Fe/H] diagram.
Aims. Precise relative ages and orbital parameters are determined for 67 halo and 16 thick-disk stars having metallicities in the range −1.4 < [Fe/H] < −0.4 to better understand the context of the two halo populations in the formation and evolution of the Galaxy.
Methods. Ages are derived by comparing the positions of stars in the log Teff–log g diagram with isochrones from the Y2 models interpolated to the exact [Fe/H] and [α/Fe] values of each star. The stellar parameters have been adopted from the preceding spectroscopic analyses, but possible systematic errors in Teff and log g are considered and corrected. With space velocities from Paper I as initial conditions, orbital integrations have been carried out using a detailed, observationally constrained Milky Way model including a bar and spiral arms.
Results. The “high-alpha” halo stars have ages 2–3 Gyr larger than the “low-alpha” ones, with some probability that the thick-disk stars have ages intermediate between these two halo components. The orbital parameters show very distinct differences between the “high-alpha” and “low-alpha” halo stars. The “low-alpha” ones have rmax’s to 30–40 kpc, zmax’s to ≈18 kpc, and emax’s clumped at values greater than 0.85, while the “high-alpha” ones, rmax’s to about 16 kpc, zmax’s to 6–8 kpc, and emax values more or less uniformly distributed over 0.4–1.0.
Conclusions. A dual in situ-plus-accretion formation scenario best explains the existence and characteristics of these two metal-rich halo populations, but one remaining defect is that this model is not consistent regarding the rmax’s obtained for the in situ “high-alpha” component; the predicted values are too small. It appears that ω Cen may have contributed in a significant way to the existence of the “low-alpha” component; recent models, including dynamical friction and tidal stripping, have produced results consistent with the present mass and orbital characteristics of ω Cen, while at the same time including extremes in the orbital parameters as great as those of the “low-alpha” component.
Key words: stars: abundances / Galaxy: halo / stars: kinematics and dynamics / Galaxy: evolution
Based on observations made with the Nordic Optical Telescope on La Palma, and on data from the European Southern Observatory ESO/ST-ECF Science Archive Facility (programmes 65.L-0507, 67.D-0086, 67.D-0439, 68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585, 76.B-0133 and 77.B-0507).
© ESO, 2012
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